Spring disc energy harvester apparatus and method

ABSTRACT

An energy harvesting apparatus and method that is especially well suited for harvesting low frequency broadband vibration energy from a vibrating structure is presented. The apparatus includes a pair of disc springs that are arranged in an opposing relationship. A threaded fastening member and a threaded nut extend through apertures in each of the disc springs and enable a predetermined preload force to be applied to the disc springs. The preload effectively “softens” the disc springs, thus heightening the sensitivity of the disc springs to low frequency, low amplitude vibration energy. A piezoelectric material ring is secured to each of the disc springs. Each piezoelectric material ring experiences changes in strain as its associated disc spring deflects in response to vibration energy experienced from a vibrating structure. The electrical output from each piezoelectric material ring can be used to power or activate various forms of electronic sensors and devices, or it can be conditioned and stored in a circuit for later use.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent application Ser. No. 11/672,695 filed on Feb. 8, 2007. The entire disclosure of the above application is incorporated herein by reference.

FIELD

The present disclosure relates to energy harvesting apparatus and methods and, more particularly, to an energy harvesting apparatus and method that makes use of a spring disc, commonly known as a “Belleville” spring, to harvest vibration energy from a vibrating structure.

BACKGROUND

The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.

Electrically-powered devices require a power source. Electrical power can be supplied in a variety of ways, including through wiring from a centralized source or from a battery. Many electrical devices are used on mobile platforms, such as aircraft, aerospace vehicles, rotorcraft, etc. The wiring typically used in these applications is heavy and costly. The use of batteries requires periodic replacement and/or recharging. In addition, a battery contains corrosive materials, and this can be a factor in limiting the use of a battery in some applications. Furthermore, in some aerospace and aircraft applications such as flight testing, various forms of sensors are located in areas where it would be costly to route power wiring.

Various attempts have been made to use piezoelectric material as a component of an energy harvesting device. When piezoelectric material is strained, an electrical charge is generated through the coupling of the mechanical and electrical states of the material. The charge generated can be useful electrical energy. The development of areas and methods of harnessing this electrical energy is finding considerable interest at the present time for their potential to power various forms of sensors and electrical components, and especially in applications where it is impractical or difficult to make use of a battery and/or wiring leading to the sensor or device.

Various forms of piezoelectric devices have attempted to convert vibrating energy from a structure into useful electrical energy. However, many piezoelectric energy harvesting devices have difficulty harvesting vibration energy at low frequencies (i.e., frequencies typically less than 100 Hz). The problem with such piezoelectric devices is their lack of sensitivity to low frequency vibration energy. A device able to convert low frequency vibration energy into useful electrical energy would thus prove highly useful in a wide variety of applications where the need exists to power a remotely located sensor or other form of electronic device.

SUMMARY

The present disclosure is related to a system and method for harvesting vibration energy. The system and method is particularly useful for harvesting low frequency vibration energy, but is not limited to such, but rather is responsive to a relatively wide frequency range of vibration energy.

In one embodiment a vibration energy harvesting apparatus is provided that includes a first disc spring having an axial center and an outer peripheral area, a second disc spring having an axial center and an outer peripheral area, and an electrically responsive material secured to a surface of the first disc spring. Alternatively, electrically responsive material may be secured to surfaces of both of the disc springs. The disc springs may each comprise what is commonly known as a “Belleville” spring. Alternatively, any like disc having a generally frusto-conical shape with a spring-like quality may potentially be employed.

A support ring may be used for supporting outer peripheral areas of the first and second disc springs and holding the disc springs in facing relationship to one another. When loaded, disc springs exhibit a non-linear stiffness behavior, with regions of low stiffness. A fastening assembly is used to apply a preload force to the disc springs to soften the disc springs to a low stiffness. The apparatus may be supported from a vibrating structure via the support ring or a portion of the fastening assembly. With either mounting arrangement, the disc springs are free to move in response to vibration energy from a vibrating structure.

In one form the electrically responsive material comprises a piezoelectric ring of material that is adhered to an associated one of the spring discs. The piezoelectric material generates electrical signals in response to changes in strain as the disc flexes slightly in response to the vibration energy transmitted to it from the vibrating structure. The electrical signal generated from the piezoelectric material can be used to power an external device or even to actuate some form of actuator, sensor or other electronic or electromechanical component or it can be conditioned and stored in a circuit for later use.

The present disclosure also relates to a method for harvesting vibration energy. In one implementation the method involves securing a pair of spring discs to a vibrating structure, where the spring discs are pre-loaded with a force sufficient to deflect them to a condition of low stiffness, to thus significantly soften the spring discs. This makes the spring discs highly sensitive to low frequency, low amplitude vibration energy.

An electrically responsive material is secured to the spring disc. The material generates an electrical output signal in response to changes in strain that it experiences as the spring disc flexes in response to vibration transmitted to it from the vibrating structure. The electrical output signals from the electrically responsive material may then be used to power or actuate an electrical, electronic or electro-mechanical device.

Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way.

FIG. 1 is a perspective view an energy harvesting apparatus in accordance with one embodiment of the present disclosure;

FIG. 2 is an exploded perspective view of the apparatus of FIG. 1;

FIG. 2A is an enlarged cross sectional view showing the attachment of one of the piezoelectric material rings to its associated spring disc, as taken in accordance with section line 2A-2A in FIG. 2;

FIG. 3 is a cross-section of the assembled apparatus in accordance with section 3-3 in FIG. 1;

FIG. 4 is a simplified side view of one of the spring discs illustrating the geometry of the spring disc;

FIG. 5 is a force versus deflection curve for the spring disc of FIG. 4 illustrating the region of low stiffness which the spring disc of FIG. 4 is pre-loaded to once fully assembled;

FIG. 6 is a graph illustrating the force versus deflection curves of a pair of Bellville springs arranged in facing relationship with one another, such as shown with the apparatus of FIG. 1, and illustrating the region of low stiffness within which the springs operate;

FIG. 7 is a simplified side view of an arrangement for supporting one of the disc springs by use of a magnetic bearing; and

FIG. 8 is a simplified side view of an alternative magnetic bearing arrangement for supporting one of the disc springs.

DETAILED DESCRIPTION

The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses.

Referring to FIG. 1, there is shown an exemplary energy harvesting apparatus in accordance with an embodiment of the present disclosure. The energy harvesting apparatus 10 may be mounted to a vibrating structure 12 that vibrates at a frequency over a relatively wide frequency range (e.g., between about 10 Hz-1 KHz). The apparatus 10 is supported from the vibrating structure 12 in this example by mounting arms 15 that are secured in any suitable manner to the vibrating structure 12. Thus, the apparatus 10 receives the vibration energy from the structure 12 and vibrates in accordance with the structure. Preferably, the apparatus 10 is mounted relative to the structure 12 such that the axis of motion of the apparatus 10 is parallel to the axis of vibration being experienced by the structure 12, in this example along the axis defined by arrow 16.

The apparatus 10 generates electrical power in response to the vibration energy from the vibrating structure 12 and transmits the electrical power to a suitable power conditioning system 18, which then supplies an electrical power output 20 to an electronic or electromechanical device requiring electrical power. While the apparatus 10 is especially well suited for providing electrical power to power other electrical, electronic or electromechanical devices, it will be appreciated that the electrical output signals generated by the apparatus 10 could just as readily be used to turn on and off a sensor or other electrical, electronic or electromechanical component or be conditioned and stored in a circuit for later use.

Referring to FIGS. 2 and 3, the construction of the apparatus 10 is shown in greater detail. The apparatus 10 generally includes a first disc spring 22, a second disc spring 24 and a support ring 26 for supporting peripheral edge portions 28 and 30 of the first and second disc springs 22 and 24, respectively. The disc springs 22 and 24 may comprise well known “Belleville” springs. Alternatively, any resilient, frustoconical shaped disc is potentially useable. A power conditioning subsystem 18, as will be further described in the following paragraphs, processes the electrical signals obtained from the apparatus.

The first disc spring 22 further includes a ring of electrically responsive material 32, which in one preferred form may comprise a piezoelectric material ring. Similarly, the second disc spring 24 includes an electrically responsive material ring 34 secured thereto, which also may comprise a piezoelectric material ring. For convenience, material rings 32 and 34 will be referred to as “piezoelectric” material rings throughout the following discussion. It will be appreciated, however, that any material that is able to generate electrical signals in response to changes in strain may be used in place of a piezoelectric material. Such other materials might include polyvinylidine fluoride (PVDF) film. Each piezoelectric material ring 32,34 is further arranged coaxially with the axial center of its associated disc spring 22 or 24.

Electrical conductor 22 a is electrically coupled to the piezoelectric material ring 32 by any suitable conductive adhesive or mechanical connection, while electrical conductor 22 b may be similarly coupled to the first disc spring 22. Conductors 22 a and 22 b feed the potential difference signal generated across the piezoelectric material ring 32 to a first input channel of the power conditioning system 18. Conductor 24 a is electrically coupled to the second piezoelectric material ring 34 by any suitable conductive adhesive or mechanical connection, and conductor 24 b may be similarly conductively coupled to the second disc spring 24. Conductors 24 a and 24 b feed the potential difference signal generated across the second piezoelectric material ring 34 into a second input second channel of the power conditioning circuit 18. The power output 20 of the power conditioning system 18 represents an electrical signal that may be related to the vibration energy harvested by the apparatus 10.

Referring further to FIGS. 2 and 3, the first disc spring 22 includes an aperture 36 at its axial center, while the second disc spring 24 similarly includes an aperture 38 at its axial center. The fastening member 14 is inserted through a washer 40 a, which may be optional, through the apertures 36 and 38, through an optional washer 40 b, and is engaged with a nut 42 to hold the disc springs 22 and 24 clamped against oppositely facing edge portions 44 and 46 of the support ring 26, and in an opposing but axially aligned relationship. In this regard, it should be appreciated that edge portions 44 and 46 may include a notch or shoulder formed therein to help maintain the disc springs 22 and 24 axially aligned during an assembly procedure. The fastening member 14 also may have a length that is sufficiently long to enable it to be used to secure the apparatus 10 to the vibrating structure 12, in place of the arms 15 shown in FIG. 1. The fastening member 14 and the threaded nut 42 allow an adjustable degree of preload force to be applied to the disc springs 22 and 24 during assembly. This is advantageous, as will be explained in the following paragraphs. Depending on the thread count of the fastening member 14, and analytical or computational modelling or empirical testing, it is possible to determine that a specified number of turns of the fastening member 14 will apply a known preload force to the disc springs 22,24. The mass of the fastening member 14 and nut 42 may increase the amplitude of the motion of the apparatus 10, further increasing the sensitivity of the system to low frequency vibration energy.

With further reference to FIGS. 2 and 3, the assembly of disc spring 22 and the piezoelectric material ring 32 will be described in greater detail. Disc springs 22 and 24 may be made from spring steel or any other material having suitable resilient properties, such as carbon fiber reinforced plastic. The disc springs 22 and 24 are held by a suitable tool (not shown) in axial alignment with one another, with their outer peripheral edges 28 and 30 against the support ring 26. Using fastening member 14, a predetermined preload force is applied to the disc spring. This causes the disc springs 22 and 24 to flex (i.e., deflect) slightly. The precise preload may vary depending upon the geometry of the cross section of the disc springs 22,24. Preferably, the amount of preloading is sufficient to place the disc springs 22, 24 at the middle of a range of low stiffness. For a single disc spring 22 or 24, this range is illustrated in FIG. 5. In this example, the preloading force would be sufficient to cause a deflection of one of the disc springs 22 or 24 by at least about 0.03 inch (0.762 mm), which puts it at approximately the beginning point of the “reduced stiffness” range (i.e., point 48) on the graph of FIG. 5, and more preferably by about 0.042 inch (1.0668 mm) to place it at the midpoint of the reduced stiffness range.

With further reference to FIGS. 2 and 2A, while the disc spring 22 is held with the above-described degree of preloading force, the piezoelectric material ring 32 is adhered thereto. In one specific form of assembly, a plurality of spaced apart drops of conductive adhesive 50 are placed along the undersurface 52 of the piezoelectric material ring 32, and separated by a layer or nonconductive adhesive 54. The conductive adhesive drops 50 provide electrical conductivity between the piezoelectric material ring 32 and the disc spring 22, which allows the disc spring to conveniently act as an electrical connection to the electrode on the piezoelectric material ring 32 that is in contact with the disc spring 22. Conductors 22 a and 22 b electrically coupled to the disc springs 22, 24 allow the electrical current generated by the piezoelectric material layers 32, 34 to be transmitted to the power conditioning subsystem 18 (FIG. 1).

Nonconductive adhesive 54 is used to provide a strong bond between the piezoelectric material layer 32 and the outer surface 56 of the disc spring 22. Prior to adhering the piezoelectric material layer 32, it is also preferred to thoroughly clean the outer surface 56 of the disc spring 22, and possibly also to sand the surface 56 so that a surface is presented that will enable a strong bond to be achieved. For the conductive adhesive 50, various forms of adhesive may be used, but one suitable adhesive is CHO-BOND®, a two-part conductive epoxy commercially available from Chomerics, a company of the Parker Hannifin Corporation. The non-conductive adhesive 54 may also take a plurality of forms, but one suitable adhesive is commercially available LOCTITE-HYSOL® 9330 two-part epoxy.

Once the adhesives 50 and 54 have cured, any tooling being used to hold the disc springs 22,24 in place during the curing process may be removed. Once this manufacturing operation has been completed for both of the disc springs 22 and 24, the apparatus 10 may be assembled and the nut 42 adjustably tightened on the fastening member 14. The nut 42 is tightened sufficiently to provide a preload force that deflects each of the disc springs 22 and 24 to approximately a midpoint of its low stiffness region. The low stiffness region for one of the disc springs 22 or 24 is defined by arrow 58 in FIG. 5.

With further reference to FIGS. 4 and 5, it will be appreciated that, when loaded, the disc springs 22 and 24 of apparatus 10 will typically exhibit non-linear stiffness. The extent of this non-linear stiffness is governed primarily by the height-to-thickness ratio (h/t) of the disc spring 22 or 24. The thickness is denoted by “t” in FIG. 4, while the height is denoted by “h” in FIG. 4. It will also be appreciated that attaching the piezoelectric material ring 32 to the disc spring 22 essentially increases the effective thickness of the disc spring 22,24 and thus decreases its height-to-thickness ratio (h/t), which in turn alters the non-linearity of the force-deflection curve shown in FIG. 5. Thus, the dimensions of the piezoelectric material rings 32 and 34 will also need to be considered when tailoring the response of the disc springs 22 and 24, respectively, to place them each in their low stiffness operating region.

Still another factor that must be taken into account is the added stiffness of the piezoelectric material rings 32 and 34. Preferably, the added stiffness provided by the piezoelectric material rings 32 and 34 is accounted for by selecting disc springs 22 and 24 that have suitably high height-to-thickness ratios. Generally, the higher the height-to-thickness ratio for the disc spring, the more piezoelectric material that can be attached (i.e., the greater the thickness of the piezoelectric material layer 34 that can be used). It is also possible to use disc springs having tapering wall thicknesses. It will also be appreciated that the threaded fastener 14, the nut 42 and the washer 40 may also impact tuning of the disc springs 22 and 24, and therefore will likely need to be accounted for when setting the preload force for the disc springs 22,24.

Referring briefly to FIG. 6, the force versus deflection curves for exemplary spring discs 22 and 24 are indicated by curves 60 and 62, respectively. The reaction forces from disc springs 22,24 are in opposite directions because of the opposing configuration of the springs. The resulting region of low stiffness of the disc spring pair 22,24 is defined by portion 64 of curve 66. Again, ideally, the preload force supplied to the disc spring pair 22,24 is such as to deflect the disc springs 22,24 to a midpoint of the low stiffness range 64.

In operation, as the apparatus 10 of FIG. 1 experiences vibration from the vibrating structure, the deflection of each of the disc springs 22 and 24 within the region defined by arrow 64 in FIG. 6 causes strains to be generated within the disc springs. Analysis indicates that for this exemplary configuration, approximately 1000 microstrain is achieved for a 0.020 inch (0.508 mm) deflection of each disc spring 22 and 24. These strains are transmitted to their respective piezoelectric material rings 32 and 34 through the epoxy 50,54 (FIG. 2A) and converted into electrical energy by the piezoelectric material rings as the rings are strained.

With the apparatus 10, the opposed arrangement of the disc springs 22 and 24 allows each of the disc springs to be preloaded to its low stiffness region and the deflecting motion of the disc springs is not in anyway impeded by the motion of the other. In certain geometries and/or applications, it may be preferable to provide the support ring 26 with a height that enables each of the disc springs 22 and 24 to flex beyond its flattened position.

An alternative implementation of the apparatus 10 involves securing the apparatus 10 to a vibrating structure by using a portion of the threaded fastening member 14. The fastening member 14 would need to have a length sufficient to allow for this. With this arrangement, the “input” vibration energy would be applied to the fastening member 14, which would then cause flexing of the disc springs 22 and 24. One advantage of this implementation would be that the mass of the support ring 26 (FIG. 1) could be used to enhance the amplitude of the vibrating motion of the apparatus 10, and thus even further increase the sensitivity of the apparatus 10 to low frequency vibration energy.

The disc springs 22 and 24 are able to respond to a wide frequency range of low amplitude vibration energy. The apparatus 10 is responsive to a vibration energy having a frequency as low as about 5 Hz or potentially even lower. This is due in part to the low stiffness of the disc springs 22,24 when they are preloaded. Some forms of vibration energy harvesting devices have relied on biasing a support member to a “buckling” point to soften the biasing member, and thus heighten its responsiveness to vibration energy. However, buckling is highly sensitive to boundary conditions that can sometimes be difficult to closely manage during a manufacturing process. The low stiffness of the disc springs 22 and 24 can be achieved in large part because of their natural force-deflection characteristics, arising from their axisymmetric geometry. This helps to make the disc springs 22 and 24 less sensitive to boundary conditions than devices that employ buckling to soften the support element.

Referring now to FIGS. 7 and 8, two alternative arrangements are shown for supporting the disc springs 22,24 to minimize the friction between the inner and outer edges of the disc springs with the components with which they are in contact. It will be appreciated that minimizing the friction enhances the ability of the disc springs 22,24 to respond to low frequency and/or lower amplitude vibration from a vibrating structure. In FIG. 7 a permanent magnet 70 is bonded or otherwise secured in an outer peripheral edge 72 of a disc spring 74, which may be identical or similar to disc springs 22 and 24. More preferably, a plurality of permanent magnets 70 will be secured about the peripheral edge 72 of the disc spring 74 and spaced apart from one another. An inner peripheral edge 76 may correspond to an edge of disc spring 22 immediately adjacent the aperture 36. A magnet 78 is attached to a supporting structure 80, which may or may not correspond to the support ring 26 shown in FIGS. 1 and 2, while another permanent magnet 82 is coupled to a structure 84 adjacent the inner peripheral edge 76. The permanent magnets 70 and 76 are further arranged such their negative poles face the negative poles of magnets 78 and 82, respectively. In this manner the magnetic forces from the magnets pairs 70/78 and 76/82 repel, thus preventing physical contact of the magnets of each pair 70/78 and 76/82 when the disc spring 74 is preloaded.

Another arrangement for forming a magnetic bearing is shown in FIG. 8. The disc spring 74 includes two permanent magnets 70A and 70B formed in its outer peripheral edge 72, while the inner peripheral edge 75 includes a permanent magnet 76′. A fastening structure 84 includes a permanent magnet 82′ formed therein. Preferably, a plurality of pairs of magnets 70A,70B, and a plurality of magnets 76′ will be spaced apart around the peripheral edges 72 and 75, respectively, of the disc spring 74. The magnets 70A,70B and magnets 76A,76B are arranged so that their magnetic lines of flux repel. Furthermore, magnets 76′/82′ have their magnetic poles arranged so that they repel. Peripheral edges 72 and 75 will thus be supported in a non-contact arrangement.

While various embodiments have been described, those skilled in the art will recognize modifications or variations which might be made without departing from the present disclosure. The examples illustrate the various embodiments and are not intended to limit the present disclosure. Therefore, the description and claims should be interpreted liberally with only such limitation as is necessary in view of the pertinent prior art. 

1. An energy harvesting apparatus, comprising: a first disc spring having an axial center and an outer peripheral area; a second disc spring having an axial center and an outer peripheral area; an electrically responsive material secured to a surface of said first disc spring, said electrically responsive material operating to generate an electrical output signal in response to changing levels of strain experienced by said electrically responsive material, in response to flexing motion of said first disc spring; a support member for supporting said outer peripheral areas of said first and second disc springs and holding said disc springs in facing relationship to one another; and a fastening assembly for applying a preload force to said disc springs to soften said disc springs, said fastening assembly adapted to be secured to a vibrating structure such that vibration energy from said structure is transmitted to said disc springs, causing flexing of said disc springs.
 2. The apparatus of claim 1, further comprising an electrically responsive material secured to said second disc spring.
 3. The apparatus of claim 1, wherein said electrically responsive material comprises a ring of piezoelectric material adhered to said first disc spring coaxially with said axial center of said first spring disc.
 4. The apparatus of claim 1, wherein said disc springs each include an aperture at said axial center thereof.
 5. The apparatus of claim 4, wherein said fastening assembly includes a threaded bolt extending through said apertures in said disc springs, and a threaded nut secured to said threaded bolt.
 6. The apparatus of claim 1, wherein at least one of said disc springs is comprised of spring steel.
 7. The apparatus of claim 1, wherein said preload force is sufficient to place said disc springs in a condition of low stiffness.
 8. The apparatus of claim 7, wherein said preload force deflects said disc springs to positions where said disc springs are at an approximate midpoint of a range of low stiffness.
 9. An energy harvesting apparatus, comprising: a disc spring having an axial center and an outer peripheral area; an electrically responsive material secured to a surface of the disc spring, said electrically responsive material operating to generate an electrical output signal in response to changing levels of strain experienced by said electrically responsive material, in response to flexing motion of said disc spring; a support member for supporting said outer peripheral area of said disc spring, said support member further being supported, relative to a structure, to receive vibration energy experienced by said structure, so that said vibration energy causes flexing of said disc spring; and a fastening assembly for securing said disc spring to said support member with a predetermined preload force directed along said axial center of said disc spring sufficient to soften said disc spring to promote flexing thereof in response to said vibration.
 10. The energy harvesting apparatus of claim 9, wherein: said disc spring includes an aperture formed at an axial center thereof; said support member includes an aperture at an axial center thereof; and said fastening assembly includes a threaded bolt that extends through said apertures, and a threaded nut that enables said preload force to be applied to said disc spring.
 11. The energy harvesting apparatus of claim 10, wherein said electrically responsive material comprises a piezoelectric ring having an aperture formed at an axial center thereof.
 12. The energy harvesting apparatus of claim 11, wherein said piezoelectric ring is adhered to said surface of said disc spring.
 13. The energy harvesting apparatus of claim 9, wherein said support member comprises another disc spring having a peripheral area, and wherein said peripheral areas of said disc springs are arranged in opposing relationship to one another.
 14. The energy harvesting apparatus of claim 9, wherein said disc spring is comprised of spring steel.
 15. A method for forming an energy harvesting device, comprising: a) providing a disc spring; b) supporting an outer peripheral edge of said disc spring; c) applying a pre-load force to an inner peripheral edge of said disc spring directed along an axial center of said disc spring; d) while said pre-load force is being applied, using an adhesive compound to adhere a piezoelectric material to said disc spring; e) waiting a predetermined time until said adhesive compound has cured; f) securing said disc spring to a support element using a fastening assembly; and g) using said fastening assembly and said support element to apply a predetermined preload force to said disc spring that causes a degree of deflection of said disc spring, said deflection being sufficient to place said disc spring in a condition of reduced stiffness.
 16. The method of clam 15, wherein using said fastening assembly comprises using a threaded bolt having a threaded nut.
 17. The method of claim 15, further comprising repeating operations b) through g) for a second disc spring and arranging said disc spring and said second disc spring in opposing relationship.
 18. The method of claim 15, wherein using an adhesive compound includes using a first electrically conductive, adhesive compound and a second, non-conductive adhesive compound.
 19. A method for harvesting vibration energy from a vibrating source, comprising: securing a pair of disc springs to the vibrating source, where the disc springs are held in opposing relationship and pre-loaded with a force sufficient to substantially soften the disc springs and to make the disc springs sensitive to low frequency, low amplitude vibration energy; securing a material to a first one of the disc springs, where the material generates an electrical output signal in response to changes in strain that is experienced as said first disc spring flexes in response to vibration transmitted from said vibrating structure; and receiving electrical output signals from said material as said one disc spring flexes during vibration of said structure.
 20. The method of claim 19, further comprising: securing a material to a second one of said disc springs to generate electrical signals in response to changes in strain experienced by said second one of said disc springs. 